Polyoxymethylene resins have been widely used not only for various mechanism parts but also office automation equipment and the like as an engineering resin having balanced mechanical properties and excellent wear resistance. However, the polyoxymethylene resin itself cannot give sufficient sliding property to molded articles, and hence a low molecular weight lubricant such as a fatty acid ester, polyethylene glycol, silicone or the like has been added to the polyoxymethylene resin.
As a technique of using a polyoxymethylene resin for such a ramp material for hard discs, JP-A 2001-297548 (US 2001-040769 A1) discloses use of a resin material having a tensile elongation of not less than 30%, and a polyacetal resin is used as the resin material. Furthermore, as a lubricant there are cited fluorine compounds such as PTFE, phosphazene type lubricants, carnauba wax, polyethylene wax, metal soaps, and the like.
When a usual polyoxymethylene resin containing no lubricant was used as a hard disc ramp material, no deterioration of sliding property was found due to washing with a solvent (which includes halogen-containing solvents such as trichloroethylene, trichloroethane, and various chlorofluorocarbons, aliphatic and aromatic hydrocarbons, alcohols, liquefied gases such as liquefied carbon dioxide gas, surfactant-containing water and pure water), but there was a problem that the level of the friction coefficient was high. On the other hand, a polyoxymethylene resin material having incorporated therein a lubricant such as a fatty acid ester, polyethylene glycol, silicone or the like to improve frictional abrasion, had excellent sliding property before solvent washing but showed remarkable deterioration of sliding property after solvent washing. Studies to prevent such deterioration of sliding property due to solvent washing have not been carried out.
In Japanese Patent Application 2001-535463 (the corresponding publication, WO 01/32775), the present inventors disclose that when a polyolefin resin grafted with a silicone gum is used as a lubricant, the sliding property of molded articles is not deteriorated even after washing with a solvent in the course of dry cleaning or the like. However, such a lubricant contains a silicone component which gives an adverse influence to the magnetic record of hard discs, and hence it was impossible to use such a lubricant as a ramp material for hard discs. Therefore, there is a need for a ramp material which does not contain a silicone component and which does not show deterioration of sliding property (friction coefficient/abrasion amount) due to solvent washing.
In this connection, in the case of phosphazene type lubricants, carnauba wax, polyethylene wax, and metal soaps disclosed as a lubricant in the above JP-A 2001-297548 (U.S. 2001-040769 A1) the sliding property was deteriorated by solvent washing, and in the case of fluorine compounds such as PTFE deterioration of the sliding property due to solvent washing was not found, but improvement in the friction coefficient was insufficient.
Furthermore, it is disclosed in WO 99/43751 to add a polyolefin resin (b-1) used as the component (B) of the present invention to a polyoxymethylene resin, and it is disclosed in JP-A 2002-20578 to add a polymer (b-2) obtained by polymerizing an isocyanate compound and a polyalkylene oxide to a polyoxymethylene resin. However, it is not disclosed at all therein to use these materials as a ramp material for hard discs.